Abstract:

By replacing coal with renewable fuels, such as wood, the consumption of coal and the amount of fossil fuel emissions can be reduced. The wood fuels need however processing before combustion in pulverized fuel firing boilers. The aim of this work is to study the applicability of the DustComb-process as a pretreatment method for wood fuels.

The principle of the DustComb-process is to defibrate the woody feedstock with a mechanical defibrator, and the produced fines are dried with a flash dryer integrated into the process. The dried fines are either fed into a pulverized fuel burner or stored for later use. In this work, the co-firing of biomass in a pulverized coal boiler is discussed, and the principles of mechanical defibration and flash drying are explained. In the experimental part of this study, the influence of the disc clearance on the defibrator’s specific energy consumption is determined. The disc clearance in the defibration tests are varied between 0.60 and 1.80 mm. Also the effect of the particle size on the drying rate is studied. In the calculations part, the costs of producing the DustComb-fuel are estimated and their dependence on the market price of electricity, the price of heat, and the price of the feedstock are evaluated. The costs are further compared with the market prices of coal and wood pellets.

The experimental results show that the energy consumption of the defibration starts to increase rapidly when the disc clearance is narrowed from 1.00 mm. According to the test results, the studied particle size changes do not have any remarkable effect on the drying rate of the material. A significant observation is that a great amount of the wa-ter in the wood is evaporated already during the defibration (16–25% of the initial moisture). The calculation results show that the production of the DustComb-fuel is economically viable when compared to the price of coal, and especially when com-pared to the price of wood pellets. The results also show that the costs of the energy needed for the DustComb-process are low compared to the price of the raw material.